1. Field of the Invention
This invention is concerned with a system for expanding the stereo base of acoustic stereophonic diffusion apparatus.
When the distance between the two loudspeakers of an acoustic stereophonic diffusion apparatus is relatively small (e.g. 60-70 cm.) the stereophonic effect is almost imperceptible, or anyway inadequate, for listeners positioned some distance away from the loudspeakers. In order to simulate the effect that would be obtained with speakers set farther apart, it is known to process the acoustic input signals of the system, to enhance their stereophonic characteristics, thus performing the so-called "expansion of the stereo base".
The conventional method for achieving said aim is to use a "recursive" technique, in which a crosstalk in phase opposition is created between the two acoustic channels (left and right), usually by subtracting the output signal of each channel from the input signal of the other channel, with a suitable amplification or attenuation of said subtracted signals. In practice such crosstalk is performed only on a part of the total band of the signal, in order to avoid problems related to the propagation of acoustic waves and to the physiology of listening, which are known to persons skilled in the art.
Therefore the crossing paths of the signals being fed back to the input include band pass filters, and correction filters are often included in the two direct channels as well. Both the two direct channels and the crosstalk paths can therefore be regarded as filters (active or passive), and when, in the following disclosure reference is made to amplifiers, everywhere it should be understood that the amplification provided may incorporate a desired filtering as well.
Systems of the above mentioned kind are described, e.g., in "DIGIT 2000--VLSI Digital TV System", ITT Semiconductors, Publication Order No. 6251-190-2E, page 1.13, August 1982, or in "The German Systems and Integrated Circuits for `High-Quality` TV Receivers", by U. Buhse, in IEEE Transactions on Consumer Electronics, Vol. CE-28, No. 4, page 489, November 1982. Such known solutions require four amplifier (and/or filtering) blocks, i.e. two for the direct channels (i.e. for driving the downstream stages of the system), and the other two for the signals being fed back to provide the expansion. The complexity of such blocks depends on the complexity of the filtering functions which it is desired to obtain. Furthermore, is it is desired to make the degree of expansion programmable, the technique is critical because it requires the parallel adjustment of the gain and/or the filtering in two different blocks in an identical degree.